Aqueous adhesives have been widely used for bonding paper and plastic labels to glass and plastic surfaces. Bottle labeling adhesives are a subset of such aqueous adhesives. Labels are applied to beverage containers by high speed machines which transfer the adhesive to the label. Once coated with adhesive, the label is contacted with the container for permanent adhesion. Mechanical labeling via automated high speed machines requires the coordination of the criteria of numerous physical properties during the processing stage and, on the finished labeled product. During processing the adhesive must be capable of flowing, i.e., it should have a viscosity preferably within a range from about 20,000 to 200,000 mPas, have a high initial adhesion, which prevents undesired displacement of a freshly attached label on a base, and able to be processed on machines working with high-speed emulsions. Natural polymers such as starch and casein are often used as the base polymer in bottle labeling adhesives. Starch and casein based adhesives can be formulated to offer advantages such as machinability, high wet strength, and ice water resistance.
Recent ecological pressure has reversed the trend toward “non-returnable” bottles and reemphasized the need for “returnables”. As a result, many of the prior requirements for bottle labeling adhesives have been altered. One of the primary requirements for returnable bottles is that the labels be easily removed from the bottle prior to reutilization. The inability to remove the labels has made recently developed bottle labeling adhesives unacceptable for commercial operations. Casein based adhesives have been widely used since the dried adhesive film is responsive to caustic cleaning solutions. Starch based adhesives can also be used; however these adhesives are more difficult to clean with a caustic wash.
A current trend in the bottle industry is to use clear labels, and as such, they too impose new requirements of the adhesive. Many of the current adhesives which can be used in producing removable, clear bottle labels have the problem that they discolor (blush), when wet. Bottles often are placed in ice baths and, not only must the adhesives be ice-proof they also must be blush resistant. Adhesives, therefore, must possess an optimum balance in properties of being blush resistant, cold water resistant, and yet, allow the labels to be easily removed or stripped from the bottles.
Starch glues or dextrin glues have been widely used as adhesives for bottle labeling. However, they are not suited for producing blush resistant labels. Starch and ammonium salts of styrene-maleic anhydride resins have been used as they exhibit superior ice-proof properties. However, the bond formed by many of these adhesives is so strong that removal of the label, even upon soaking in hot alkaline water, is difficult or impossible.
Representative patents illustrating aqueous adhesive compositions useful for bottle labeling include:
U.S. Pat. No. 3,939,108 discloses a cold water resistant adhesive which has an optimum balance of cold water resistance and removability when employed for labeling of bottles. The adhesive comprises a mixture of starch, starch-maleic anhydride, peptizer, water and some alkali.
U.S. Pat. No. 4,462,838 discloses aqueous adhesives based upon starch derivatives for application in mechanical bottle labeling on high speed machines, particularly glass ware. The adhesives are comprised of a hydroxyalkyl ether of oxidized starch and a water-soluble synthetic polymer, casein or starch derivative. Examples of water-soluble polymers are polyvinyl pyrrolidone, vinyl acetate copolymers, and acrylic acid copolymers.
U.S. Pat. No. 5,455,066 discloses an aqueous adhesive based upon acid precipitated casein, water-soluble extenders, natural or synthetic resin acids, resin alcohols, and so forth for bottle labeling. Extenders are based upon copolymers of acrylic acid, acrylamides, starches, starch ethers and the like.
U.S. Pat. No. 6,590,031 discloses a pressure sensitive adhesive comprised of a polymer of an alkyl acrylate, carboxylic acid and styrene formed by polymerizing the monomers in the presence of an anionic surfactant and redox type free radical initiator. The polymers find use in clear label applications, making films, etc, and it is reported they exhibit excellent adhesion to hot water and enhanced resistance to water-whitening.
WO 03/029,378 discloses a water based adhesive that is suited for bonding plastic labels onto glass containers. The adhesive comprises a starch component and a gelatin compound.
WO 01/98422 discloses a water-based bottle labeling adhesive that promotes adhesion of paper or polysubstrates to plastic or poly-coated glass surfaces. The adhesive comprises from 30 to 80% of a synthetic base polymer, e.g., polyvinyl pyrrolidone, polyacrylic acid derivatives, carboxylated ethylene-vinyl acetate polymers, acrylic polymers, a tackifier, and additives and water. Examples of crosslinkers for the adhesives include zinc oxide, glyoxal and the like.